In production of silver halide photographic emulsions, chemical sensitization is conventionally carried out for the purpose of increasing the intrinsic sensitivity of silver halide. Typical chemical sensitization processes, including a sulfur sensitization process, a noble metal sensitization process and a reduction sensitization process, are applied alone or in combination. These sensitization processes have been applied not only to "negative emulsions" wherein latent images are mainly formed on the surface of silver halide grains but also "internal latent image type emulsions" wherein latent images are mainly formed in the inside of silver halide grains.
The internal latent image type emulsions have been utilized as direct positive emulsions (direct reversal emulsions) for forming positive images directly, i.e., without carrying out reversal processing.
For example, U.S. Pat. No. 4,431,730 discloses a process for producing an internal latent image type core/shell emulsion useful as a direct positive emulsion. The silver halide grains are composed of a silver halide core which is doped with a metal ion or chemically sensitized or is both, and a silver halide shell which covers at least a sensitivity speck of the core (hereinafter referred to as "core/shell silver halide grains"). In this process, the surface of the silver halide grains contained in an internal latent image type emulsion is chemically sensitized under the presence of a polymer such as poly(N-vinylpyrrolidone), poly-(N-vinyloxazolidone), vinyl alcohol-N-vinylpyrrolidone copolymer or N-vinylpyrrolidone-vinyl acetate copolymer.
Further, Japanese Patent Publication No. 34213/77, corresponding to U.S. Pat. No. 3,761,276, discloses an internal latent image type emulsion useful as a direct positive emulsion. In this emulsion, the inner part of the silver halide grains contains a doping agent and the surface of the grains is chemically sensitized. Such an emulsion is also disclosed in U.S. Pat. No. 3,317,322 to Porter et al.
However, in processes for forming direct positive images using an emulsion in which the surface of internal latent image type core/shell silver halide grains is chemically sensitized, increased sensitivity of the direct positive emulsion leads to the serious disadvantage that re-reversal negative images are formed by exposure to light at a high illuminance for a short time. The re-reversal negative images caused by exposure to light at a high illuminance for a short time appear as prominent white spots on the image, in the case of exposure using a flash bulb, which spoil the quality of the photograph. Therefore, it is necessary in practical use to prevent the formation of re-reversal images as far as possible.
In order to prevent formation of re-reversal negative images caused by exposure to light at a high illuminance for a short time, the surface of internal latent image type core/shell silver halide grains should not be chemically sensitized. However, in this case the emulsion is not suitable in practical use because of its low sensitivity.
British Patent Nos. 1,151,363 and 1,195,837 disclose processes for forming direct positive images having a high maximum density (D.sub.max) and a low minimum density (D.sub.min) without chemical sensitization of the surface of silver halide grains by using "conversion type" internal latent image emulsions produced utilizing the solubility difference of silver halide.
The first British Patent relates to an internal latent image type silver bromide emulsion produced by slowly adding an aqueous solution of potassium bromide to a silver chloride emulsion to effect conversion of halogen, and adding an aqueous solution of potassium iodide to the resulting emulsion, and it discloses that when the resulting emulsion layer is exposed to light at a high illuminance for a short time through a step wedge and processed with a surface developer while the whole surface is exposed to light (fogging processing), direct reversal positive images are obtained which have a high D.sub.max as compared with emulsions without being added iodine ion. The second British Patent relates to the same process as the first British Patent, except that a hydrazine type nucleating agent (fogging agent) is used instead of exposure of the whole surface (fogging processing).
The internal latent image type "core/shell silver halide" emulsion is quite different from the "conversion type" internal latent image emulsion in that the core/shell silver halide grain has a silver halide core which is doped with a metal dopant (other than Ag) and/or chemically sensitized, and a satisfactory direct positive image is not obtained, even if a "core/shell silver halide" emulsion is developed with a surface developer after adding iodine ion and carrying out exposure of the whole surface or using a nucleating agent.